Should an Underhive Nemesis be killed (i.e., suffer a Memorable Death result on the Lasting Injury table) while taking part in a scenario, it can be assumed that they survived somehow, and the Arbitrator should come up with a suitable narrative reason for their survival. Perhaps they were dragged off the battlefield by their retainers, or fell into the sump or an abyss, thought dead until they show up again. While on the surface, this might seem frustrating for the players, it is actually far more satisfying to defeat an Underhive Nemesis by foiling their plans than with a lucky lascannon shot, and having an Underhive Nemesis remain a threat until the end of a campaign gives it a sense of ongoing drama in addition to the usual struggle between the gangs.